hope I spelled it right

playing as English, France had offered me to be their Vassals (servants)

they were giving 1 of their provinces to me, and good amount of money to me in exchange to became their vassals

im cuorios what does mean being a vassal ? (in economic, political and military way)

of course I refused

but then again I was wondering, if maybe I accepted, take them one richy province, and good amount of cash and then make a rebbelion against Franch

love that feature, find it real, but can someone explain exactly the possibilites of "vassal" scenario?